338 PATTERNS AND PROBLEMS OF DEVELOPMENT 



mal determines tail development; and, as activation and dominance of 

 the tissue regenerating head decrease from X to Z, tail development 

 increases because not antagonized so strongly by dominance of the de- 

 veloping head. At more posterior levels activation and dominance of 

 regenerating tissue are not sufficient to overcome the original domi- 

 nance, and only posterior ends develop. Both heads and posterior ends, 

 however, though developing from partial transverse sections, are dorsi- 

 ventrally and bilaterally complete. 



In general, the polarity of a regenerated part, head, posterior end, or 

 appendage may coincide with, or be oblique or opposed to, that of the 

 original body; but the symmetry or asymmetry is usually in the same 

 direction as the original and may be complete, though regeneration is 

 from a partial section. If physiological axes are gradients, these features 

 of reconstitution are very simply accounted for. There is no polarity dif- 

 ferential on a transverse section; consequently, polarity in the reconstitut- 

 ing part is determined by the gradient arising in relation to activation 

 following section and outgrowth. On a section oblique to the original 

 polar axis there is a polar differential, and reconstitution in relation to 

 obhque section is often asymmetrical (pp. 50-53). Ventrodorsality and 

 dorsiventrahty do present a differential on a transverse plane of section, 

 and this must influence reconstitution from that plane; also, a partial 

 differential on the plane of section may be sufficient to initiate develop- 

 ment of the whole differential in the reconstituted part. 



THE ORIGINAL NERVOUS SYSTEM IN RELATION 

 TO NEW DOMINANCE 



Much experimental work has been done in the attempt to determine 

 whether or to what extent the nervous system of the original animal is a 

 determining or essential factor in reconstitution. The fundamental ques- 

 tion, as regards this problem, concerns the role of the nervous system in 

 determining a new dominant region, or, more specifically, the new nervous 

 system in that region. For example, do the parts of the nerve cords pres- 

 ent in a headless planarian piece play a part in determining the cephalic 

 ganglia in the regenerating head? 



In hydroid reconstitution the hydranth is determined basipetally from 

 the apical region; and since reorganization in a short piece may involve 

 the whole piece in formation of the apical portion of a hydranth, irrespec- 

 tive of level of origin of the piece, it is highly improbable that the part 

 of the nerve net originally present in the piece plays any essential role in 



